The production of higher molecular weight alcohols from lower molecular weight feedstocks has long interested organic chemists. Such reactions are of much greater interest following the 1991 amendments to the Clean Air Act. These amendments require the blending of fuel oxygenates into gasoline in non-attainment regions of the United States. The required oxygenates are intended to lower carbon monoxide and ozone pollution contributions from internal combustion engine exhausts. Two compounds particularly suited for use as gasoline oxygenates are tertiary amyl methyl alcohol ether (TAME) and methyl tertiary butyl ether (MTBE).
Certain branched-chain alcohols such as isobutanol and isoamyl alcohol are useful as precursors for TAME and MTBE. Isobutanol is useful because it can be dehydrated to form isobutylene, which subsequently can be etherified with methanol to yield MTBE. Isoamyl alcohol, a common name for an isomeric mixture of 2-methyl-1-butanol and 3-methyl-1-butanol, can be reacted with methanol to yield TAME. The expected widespread use of fuel oxygenates requires that refiners develop efficient processes for producing these oxygenates and their precursors.
Guerbert syntheses typically have been used to prepare higher molecular weight branched-chain alcohols from lower molecular weight starting materials. A representative Guerbert synthesis is disclosed in U.S. Pat. No. 4,518,810. In this example, a single alcohol feedstock is batch-reacted over a copper-nickel alkaline catalyst at temperatures from about 200 to 250 degrees Centigrade to produce a dimerized alcohol product. The product alcohol is a branched-chain dimer alcohol in which a dehydration reaction has removed the hydroxyl group from the alpha carbon of a first alcohol molecule and a hydrogen atom from a beta methylene carbon of a second alcohol molecule. The resultant alcohol has a carbon--carbon bond located between the alpha carbon of the first alcohol's carbon chain and the beta carbon of the second alcohol's carbon chain.
In addition to the dimerization reaction discussed above, Guerbert syntheses also have been used to condense together two different alcohols. In this case, at least one of the two alcohols must contain a dehydratable carbon atom at the beta position. An example of this type of reaction is that disclosed in U.S. Pat. No. 2,050,788, in which a mixture of ethyl and methyl alcohol and a stoichiometric excess of hydrogen is condensed over a MgO/CuO catalyst at temperatures between about 200 and 350 degrees Centigrade to provide a mixture of higher molecular weight alcohols.
While the foregoing Guerbert synthesis reactions can, under certain conditions, provide useful yields of certain higher molecular weight alcohols, the catalysts employed are believed to be subject to scintering under regeneration conditions and therefore not well-suited to the cyclic operation common in catalytic refinery operations. Refiners therefore continue to seek improved processes for producing fuel oxygenates and oxygenate precursors. Preferably, these processes should employ modern, commercially-available, easily-regenerated catalysts.